Despite substantial progress in the past two decades, cancer remains the leading cause of death by disease in US children between 1 and 15 years of age. Acute lymphoblastic leukemia (ALL) is the most common childhood cancer, and cure rates are approaching ~ 80% today. Unfortunately, 20% of children with ALL are not cured with current therapy, making the number of cases of relapsed ALL greater than the total number of new cases of most childhood cancers. Previous work has established that de novo drug resistance is a primary cause of treatment failure in childhood ALL. However, the genomic determinants of drug resistance remain poorly defined. The research supported by this MERIT award has focused on elucidating the genomic determinants of inter-patient differences in drug response in children with ALL, leading to a number of new insights into the pharmacogenomics of childhood ALL. Our scientific aims are focused on some of the most widely used antileukemic agents, thiopurines and glucocorticoids (with complementary studies of L- asparaginase and vincristine). The first aim is to identify genes that influence the response to thiopurines (mercaptopurine, thioguanine), using genomewide approaches for gene expression analyses of ALL cells, and SNP analyses of germline and ALL cell DNA to identify SNPs and copy number variations (CNVs) that are significantly related to drug sensitivity and response. Studies in Aim 2 are designed to elucidate these pharmacogenomic traits in T-ALL, a less common subtype that is more resistant to curative chemotherapy, and compare these findings to B-lineage ALL. Once we identify these genes or genome abnormalities (SNPs, CNVs) influencing ALL drug resistance and treatment response, we perform biochemical and genomic studies to determine the mechanism by which these genes or their variants influence the sensitivity of ALL cells to these chemotherapeutic agents. In the continuation of this research program, we are extending our findings from the last 4 years of funding by exploiting new genomewide strategies to identify genomic determinants of drug response (e.g., 1 million SNP arrays, gene expression arrays, mIRNA arrays), to determine whether CNVs or specific mlRNAs modify the expression of genes and thereby alter the sensitivity of ALL cells to these chemotherapeutic agents. Our preliminary findings have indeed revealed specific mlRNAs that influence the sensitivity of ALL cells to these agents, and we have also recently discovered that specific CNVs in ALL cells can significantly influence drug resistance. The continuation of these studies will provide important new insights into the genomic determinants of treatment failure and point to novel targets for developing strategies to overcome drug resistance in childhood ALL.